encoding/binary: added benchmarks
encoding/binary: added benchmarks

binary.BenchmarkPutUvarint32 20000000	 85.6 ns/op
binary.BenchmarkPutUvarint64 10000000	299   ns/op

R=rsc
CC=golang-dev
http://codereview.appspot.com/5148049

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang